Separator and method of separation

ABSTRACT

A method of separating a multiphase fluid, the fluid including a relatively high density component and a relatively low density component, that includes introducing the fluid into a separation region; imparting a rotational movement into the multiphase fluid; forming an outer annular region of rotating fluid of predetermined thickness within the separation region; and forming and maintaining a core of fluid in an inner region. Fluid entering the separation vessel is directed into the outer annular region and the thickness of the outer annular region is such that the high density component is concentrated and substantially contained within this region, the low density component being concentrated in the rotating core. A separation system employing the method is also provided. The method and system are particularly suitable for the separation of solid debris from the fluids produced by a subterranean oil or gas well at wellhead flow pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage patent application for PCT patentapplication serial number PCT/GB2007/002220, filed 15 Jun. 2007, andentitled Separator and Method of Separation, hereby incorporated hereinby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND

The present invention is related to a separator and a method ofseparation. The separator and method find particular use in theseparation of multi-phase mixtures, especially those containing gases,liquids and solids. The present invention is particularly suitable for,but not limited to, application in the separation of the products of oiland gas wells on land, on a platform, or especially in subsea locations.

The conventional approach to for the development of subsea hydrocarbonproducing fields is to establish a plurality of subsea wells connectedthrough a subsea infrastructure, pipelines and risers to a surfaceprocessing facility. As a result, solids in the well flow are currentlycarried in the fluid to the surface processing facility. The surfaceprocessing facility may be a floating production vessel or a platform.The surface processing facility has typically included separationequipment, to separate sand, gas and water from the oil produced fromthe wells. Gas and water recovered in this way are frequently reinjectedat the seabed into the wells. This necessarily entails pumping the gasand water back down a series of pipelines and risers to the seabed.

A new approach to this problem has been to dispense with much of theprocessing facilities at the surface and to locate the relevantinstallations on the seabed at a location adjacent to or convenient tothe production wells. The object is to remove the extensiveinfrastructure needed to bring all the produced streams to the surfaceand to return significant portions to the seabed for reinjection. Whilesuch an arrangement indeed reduces the capital expenditure and operatingcosts of the installations, it is accompanied by significant problems.

With processing equipment, such as water, gas and solid separators,situated on surface vessels or platforms, the servicing and maintenanceof the components is relatively straightforward, with access to theequipment being readily available. However, this is not the situationwith subsea installations, in particular those beyond safe diver depth.Rather, access to the equipment located on the seabed can be by eitherrobotic and remotely controlled equipment or by retrieving equipment tothe surface for repair and/or servicing. Therefore, intervention is verylimited, with the ability to access the equipment decreasing as thedepth of the installation below the surface increases. It will thus beappreciated that the failure of equipment located on the seabedrepresents a major operational problem and a high interventionexpenditure. Consequently, the frequency of component failure must beminimised, in particular if the option of a subsea installation is to beeconomically viable.

A particular problem arises as a result of solids produced from subseawells. It is frequently the case that subsea wells produce a significantquantity of solid formation material, together with the various fluidphases. The solid formation materials include coarse, medium and finesolid particles, such as grit and sand, together with very fineparticles, such as scale or silt. In some cases, the solids produced mayinclude stones, gravel and small rocks, depending upon the operationbeing performed downhole and the nature of the subterranean formation.Entrained solids leaving the wellhead are responsible for a high rate ofequipment erosion and destruction. It is known to provide surfaceinstallations with equipment for removing produced solids. Typically,the equipment employed is one or more settling vessels. The nature ofsolids settling necessarily requires that such vessels are very large insize. It would be advantageous if an effective means of separatingproduced solids from the fluid phase could be found that did not rely onsuch large equipment.

Separation installations are installed downstream of the wellhead andoperate at pressures significantly below the pressure of the fluidproduced at the wellhead. This necessitates employing one or more chokesimmediately downstream of the wellhead, in order to reduce the fluidpressure to the operating pressure of the settling equipment. However,choke components are particularly prone to damage from entrained solids.Further, the failure of the wellhead choke represents a majoroperational problem, in particular if the choke must be replaced, anoperation that requires the separation system to be shut down until therepair or replacement can be carried out, resulting in lost production.

Accordingly, there is a need for an effective system and method forseparating entrained solids from fluids produced from the well upstreamof the wellhead choke, that is at wellhead pressures.

It is known to provide surface installations with desanders. The knowndesanders comprise a single cyclone insert housed inside a vessel. Thedesanders of this kind may be employed either upstream or downstream ofthe wellhead choke and can thus operate at wellhead pressures. Whilethis arrangement represents an improvement over the arrangementsdiscussed above, a number of significant problems remain. First, theknown desanders rely upon conventional cyclone technology. Theseparation efficiency of conventional cyclone separators is at best onlymoderate, as discussed in more detail below. In particular, conventionalcyclone separators generally have a narrow range of operating fluidflowrates. While acceptable in many applications, this represents amajor restriction in their suitability for use in subsea installations.It is desirable for a subsea installation to have a wide operationalrange to handle the daily changes in operating conditions, and also becapable of meeting the well's production profile over an operatinglifetime of from 5 to 10 years. During this time, the composition andflowrate of the fluid produced from a given subsea well will vary,sometimes greatly. In is unacceptable to install in such a situation asystem that has a narrow window of operation. Accordingly, there is aneed for a system that can operate for an extended period of time over awide range of fluid flowrates and compositions.

Second, the known wellhead separators are designed for the separation ofsand, as suggested by their name. However, it is frequently the casethat a well will produce a wide range of solid material, much of whichis significantly larger in size than sand. A desander, by design, isoptimised to separate sand from the produced fluids. However, due to thelimitations of conventional cyclone separation techniques, this systemwill be inefficient in the separation of larger sized or coarser solids,allowing the larger solid particles to pass the separator and enter thedownstream equipment, with the results discussed above. Accordingly,there is a need for a separation system that can handle a wide range ofsolid particle sizes and maintain a high efficiency of separation.

Finally, as soon as fluid is produced from a well, the various phases,including liquid phases and gas, begin to separate. This naturalseparation of the various phases in the produced fluid stream isadvantageous and assists the downstream separation operations. It wouldbe most desirable if a system could be provided that efficientlyseparates a wide range of solid particles over a wide range of operatingpressures and flowrates, but without subjecting the fluid to a high rateof shear causing the already separated phases to mix and emulsify.

US 2004/0217050 discloses a solids separation system for well flowbackfor use in a subsea wellhead installation. The system uses ahydrocyclone to intermittently separate the heavy phase containingsolids from production fluid. The lighter phases are transported to thesurface for separation using conventional techniques. The heavy phasecollects in the bottom of the hydrocyclone, from where it isperiodically removed through a standard choke assembly, to reduce itspressure. The system of US 2004/0217050 does not fully address andresolve the aforementioned issues. Accordingly, there is a need for animproved separation system, suitable for use in a subsea environment,that addresses the aforementioned problems.

SUMMARY

According to a first aspect of the present invention, there is provideda method for separating solids from a multiphase fluid, the fluidcomprising a relatively high density component and a relatively lowdensity component, the method comprising:

-   -   introducing the fluid into a separation region;    -   imparting a rotational movement into the multiphase fluid;    -   forming an outer annular region of rotating fluid of        predetermined thickness within the separation region; and    -   forming and maintaining a core of fluid in an inner region;        wherein    -   fluid entering the separation vessel is directed into the outer        annular region; and    -   the thickness of the outer annular region is such that the high        density component and solids are concentrated and substantially        contained within this region, the low density component being        concentrated in the rotating core.

References herein to a ‘multiphase fluid’ are to include liquid/liquidmixtures, liquid/gas mixtures. In addition, the term includes mixturesof components that comprise solid materials, provided that the bulk ofthe mixture is one or more fluid phases. An example of such a multiphasefluid is a slurry or suspension of solid particles and solid material ina liquid and/or gas stream.

References herein to ‘downstream’ are to a direction extending from theinlet of the multiphase fluid to an outlet through which a solids-richstream is removed and recovered.

In a conventional cyclone separator, a multiphase fluid is directed intoa vessel, where a general circulating or rotating flow pattern isinduced. Heavier phases or components, that is those with relativelyhigher densities, are concentrated in the outer regions of the rotatingmass, while the lower density components tend to concentrate in theinner circulating regions. The fluid entering the vessel from the inletforms a rotating high pressure band from where it may split and followone of three possible paths, as the incoming fluid meets the fluidalready circulating within the vessel. In particular, fluid may leavethe main flowpath and pass towards an inner rotational region.Alternatively, fluid may leave the main flowpath and rise upwards,becoming partially trapped. Finally, fluid may leave the main flowpathand move downwards, relative to the major fluid flow. While the generaltrend within a cyclone is for heavier fluid and material to concentratein the outer regions of the vessel and for the lighter fluid andcomponents to collect in the central region, the tendency of the fluidto split into various paths causes cross-contamination of the alreadyseparate material. This leads to a reduction in the efficiency of thecyclone. The different flow paths will compete for priority, which willbe determined by the hydraulic flow parameters of the fluid, that isflow rate, pressure, and relative densities of the different phases orfractions. Accordingly, the design of a cyclone is very dependent uponthe particular flow rate and fluid parameters. Operation of a cycloneseparator outside these design parameters will reduce the overallseparation efficiency and the performance of the separator will fallsignificantly. As noted above, such a separator is unsuitable for use insituations where a long operational lifetime is required with a widerange of flow rate and hydraulic parameters to be accommodated, such asthe separation of solid material from fluids produced from a subseawell.

It has been found that an improved separation of a multiphase fluid canbe achieved when the higher density component of the fluid is confinedto and caused to rotate in an outer annular region of the separationregion, with the inner region or core being maintained and occupied bythe lower density component. It is found that the higher densitycomponents, in particular solid particles, are concentrated in therotating outer region, with a good separation of the higher densitymaterial occurring from the relatively lower density components, whichcollect in the inner region. By keeping the higher density components inthe rotating outer annular region, they are subjected to maximumrotational forces, which serves to improve the separation. The innerregion is separated from the outer annular region by an interface,across which components will migrate. In particular, lighter componentswill leave the outer annular region and pass into the core region. Incontrast, the heavier components, in particular solids, are caused toleave the core region and collect in the outer annular region. In thecase of a multiphase fluid comprising components with markedly differentdensities, in particular liquids and gases, the interface between theinner core region and the outer annular region will form a transitionzone.

The method of the present invention is applied in the separation ofsolid materials from multiphase fluid streams. The method may be appliedin the separation of immiscible liquids of different densities. Themethod may be applied to separate a combined liquid and gas stream intoits separate phases. The method of the present invention is particularlysuitable in the separation of multiphase fluid streams comprising solidparticles, liquid and gas. The method is especially suited to theremoval of solid particles from the fluid stream produced from asubterranean well, in particular a well producing oil and gas. Themethod is also suitable for the separation of such a stream thatcontains water. In such a case, the method operates to remove the solidparticles from the fluid phases and initiate or begin the separation ofindividual liquid and gas phases.

The method of the present invention is particularly suitable for theseparation of solid material from a fluid stream comprising both liquidand gas. This is typically the composition of a stream produced by asubterranean oil or gas well. When the stream contains a significantquantity of gas, the inner region consists almost entirely of gas, theaction of which is to prevent liquid and solid components from leavingthe outer annular region, resulting in a very high solids removalefficiency. This leads to a further aspect of the present invention,which provides a method for separating solid particles from a multiphasefluid stream, the fluid stream comprising a liquid component and a gascomponent, the method comprising:

-   -   introducing the fluid stream into a separation vessel;    -   imparting a rotational movement into the fluid;    -   forming an outer annular region of rotating fluid of        predetermined thickness; and    -   forming and maintaining a core of low density fluid in an inner        region; wherein    -   liquid and solid particles entering the separation vessel are        directed to the outer annular region; and    -   the thickness of the outer annular region is such that the solid        particles are concentrated and substantially contained within        this region.

The solid material is removed from the rotating annular region bytechniques described hereinafter. The outer annular region is maintainedwith a thickness sufficient to entrap and hold the solid particles.

The thickness of the outer annular region is determined by the overalldimensions and shape of the opening or inlet through which the fluid iscaused to enter the separation region. In particular, the thickness ofthe outer annular region is dependant upon the height and width of theinlet. The angle of the inlet determines the angle at which the fluidstream enters the separation region, in turn determining the pitch ofthe helical flow path followed by the fluid stream in this region. Inthis respect, references to an angle are to the angle with thelongitudinal axis of the separation region. These are selected to causethe fluid to enter and form the outer annular region. As the dimensionsof the inlet are fixed, the volumetric flowrate of the fluid feed streamentering through the inlet determines the velocity of the fluid streamand in turn determines the initial rotational velocity of the fluid inthe outer annular region.

Preferably, the inlet is arranged to introduce the fluid stream somedistance from the upstream end of the separation region. In this way, aspace is left between the upstream boundary of the outer annular regionand the end of the vessel containing the separation region. This allowsany high density fluid that splashes from the outer annular region canreturn, typically by gravity, to the rotating fluid stream and beentrapped. In this way, erosion of the end wall of the vessel containingthe separation region by high density fluids and entrained solids isminimised.

As noted above, it is frequently the case that subsea wells produce asignificant quantity of solid formation material, together with thevarious fluid phases. The solid formation materials include coarse,medium and fine solid particles, such as grit and sand, together withvery fine particles, such as scale or silt. In some cases, the solidsproduced may include stones, gravel and small rocks, depending upon theoperation being performed downhole and the nature of the subterraneanformation. The method of the present invention is effective in removingsolid material, such as grit, sand and the like in all size ranges, fromthe production fluid. However, the separation efficiency of the methodis such that it will also operate to remove larger solid particles thatmay be produced by a well. Such particles include well debris, forexample metal particles, resulting from drilling or finishingoperations, as well as debris resulting from equipment damage orfailure. In the case of failure of downhole equipment, substantial solidparticles, typically metal particles, may be produced with the wellfluid. It will be appreciated that such debris can cause significantdamage to downstream equipment, such as chokes and the like, if notremoved.

Solid particles removed from fluid streams by the method of the presentinvention may range from a few millimetres in diameter to severalcentimetres. Sand and gravel particles range from 1 mm to from 5 to 10mm. Larger debris may include objects in the range of from 1 to 5 cm orlarger. It is an advantage of the method of the present invention thatsuch a large range of solid particles may be separated from the fluidstream.

A particular advantage of the method of present invention is that it canbe operated over a wide range of flowrates, that is has a largeturn-down ratio. The method may be applied in a single separationapparatus and operate at flowrates in the range of up to 25,000 barrelsper day (BPD), in particular from zero, more particularly from 5,000 upto 25,000 BPD. That is, the method may be operated with flowratefluctuations as great as 500%, without significant reductions in theseparation performance of the system. The method preferably operates atflowrates of up to 25,000 BPD. Higher flowrates are possible, forexample of from 1,000 to 100,000 BPD, with the separation apparatusbeing sized accordingly. However, the performance of the method inremoving the smaller solid particles may be reduced. Preferred operatingflowrates are in the range of from 2,000 to 25,000 BPD, with flowratesdown to zero being operable. At lower flowrates, the separation of thecomponents may occur as a result of gravity separation effects as thedominate separation mechanism. At higher flowrates, the dominantseparation mechanism will be due to rotational movement of the fluidcomponents.

The single separation apparatus can be sized for volume throughput, inorder to cater for a particular fluid flowrate. Alternatively, theapparatus may be sized according to a desired quality of separation.Therefore, to achieve both a desired separation performance and a rangeof fluid volumetric flowrates, a plurality of apparatus of differentsizes may be applied. In this way, units of different sizes may beoperated in different combinations, by means of a suitable selectiveflow-switching means, to accommodate a range of flowrates and separationduties.

In order to impart the rotational flow pattern to the stream and createthe rotating annular region, it is preferred to use a tangential entryof the fluid stream into the separation region. Such techniques areknown in the art of cyclone separation. However, in order to ensure thatthe annular region is of sufficient depth and possesses a regular flowpattern, it is preferred that the tangential entry is at an acute angleto the longitudinal axis of the separation region. In this way, thefluid stream entering the region is caused to rotate around the innerwall of the region and not collide with the incoming stream. The overalleffect is to create a helical flow pattern within the rotating annularregion, with the components in this region rotating around the centralcore of low density components. The angle at which the fluid isintroduced into the vessel is preferably selected so that the fluidrotates in a helical path that allows the fluid after one revolution toclear the inlet jet and not impact with the incoming fluid. Typically,the angle is in the range of from 45° to 85°, more preferably from 60°to 85°, to the longitudinal axis of the separation region. Thedimensions of the inlet are selected according to the generalspecification of the well fluid and the fluid retention time required inthe outer annular region in order to allow the solids to migrate to theouter wall of the vessel.

In one preferred arrangement, the portion of the separation regionadjacent the fluid inlet is provided with a guide surface, such as in awall assembly, shaped to induce the incoming fluid stream into a spiralor helical flow pattern. This wall assembly may be disposed between theinlet and the upstream end of the separation region, in order to preventfluid entering the separation region from flowing towards the upstreamend. The wall assembly preferably has a form that induces a helical flowpattern in the incoming fluid stream, such that fluid that has enteredthe separation region is caused to move in a generally downstreamdirection sufficient so that the fluid completing its first rotationwithin the separation region is not contacted by the incoming fluidstream. The wall assembly is preferably provided with one or morehelically extending surfaces, the surfaces extending generally in thesame helical pattern as the desired helical flow pattern to be inducedin the incoming fluid stream. A suitable wall assembly is known in theart and is disclosed in GB 2,353,236.

It has been found that the use of a guide having one or more helicallyextending guide surfaces as hereinbefore described allows the angle ofthe inlet conduit be oriented closer to the perpendicular to thelongitudinal axis of the separation region.

The inlet for the fluid may be any suitable shape. Circular inlets arewell known and may be used in the method of the present invention.However, preference is given to inlets having a rectangularcross-section and opening for the incoming fluid stream. The rectangularinlet helps to generate a uniform helical flow pattern in the outerannular region. In addition, the rectangular inlet provides a betterinterface between the incoming fluid and the inner wall of theseparation region, resulting in a more stable flow regime in the outerannular region.

The rectangular inlet preferably has its longest side parallel to thelongitudinal axis of the separation region. The height of therectangular opening of the inlet, that is the dimension in the directionparallel to the longitudinal axis of the separation region, incombination with the angle of the inlet determines the pitch of thehelical flow pattern within the outer annular region. The width of therectangular opening of the inlet, that is the dimension perpendicular tothe longitudinal axis of the separation region, will determine thethickness of the outer annular region.

As the fluid stream moves helically within the outer annular region(typically downwards within a generally vertically arranged separationvessel), the solid particles will collect at the wall of the separationregion in the outermost portion of the annular region. The fluid phases,being less dense than the solids, will migrate to the innermost portionof the annular region, in which portion the fluid phases will separateinto the more dense phase, which will remain in the annular region, andthe less dense phase, which will migrate into the central core. Thiseffect is particularly marked when the fluid stream contains a gaseousphase, in which case the gas quickly leaves the rotating fluid in theouter annular region.

As the fluid stream in the outer annular region progresses, it will losemomentum, the effect of which is to cause the outer annular region tobecome thicker. As the flowpath of the fluid stream is followed, theannular region increases in thickness, until it reaches the centre ofthe separation vessel. This effect causes the interface of the leastdense fluid of the inner core and the denser fluid retained in the outerannular region to adopt a convex profile in the downstream direction.The precise form of this profile and the overall form of the interfacebetween fluids in the core and fluids in the outer annular region willdepend upon such factors as the density of the fluid stream and theinlet velocity of the stream as it enters the separation vessel. It willthus be appreciated that the profile of the interface and the overallshape and dimensions of the core region will vary as the parameters ofthe fluid stream change, such as will happen over time for a fluidstream produced by a subterranean well. As noted above, it is asignificant advantage of the method of the present invention thatchanges in the shape and dimensions of the core region, and hence theproperties of the inlet stream, can be readily accommodated without anychange in the separation apparatus being required.

Conventional cyclone separators are provided with an outlet for fluid toleave the separation vessel. The outlet is typically arranged coaxiallywithin the vessel as an exit or dip pipe, through which fluid may leavefrom the central most region of the vessel. However, such an arrangementcannot be applied in the case of the present invention. In particular,it is not possible to remove fluid directly from the core region in thisway, as this region must be allowed to form and be maintained. Rather,it is necessary that fluid removal from the core region is carried outin a controlled manner so as to maintain the core region intact. Anumber of alternative designs of fluid removal may be employed, asfollows.

In a first embodiment, a fluid collection region is provided downstreamof the core region, having a fluid outlet. Some fluid will leave thecore region by means of displacement. However, by appropriate design ofthe fluid outlet, it is possible to establish a steady-state conditionunder which the core region is maintained and fluid from the core regionis caused to exit via the outlet opening downstream of the core regionitself. With such an arrangement, the fluid exiting through the outletwill be a mixture of the high density fluid and the low density fluid.This combined fluid stream may be passed to a further separation processin order to separate the various fluid phases. An example of such aseparation process is a gas/liquid separator.

Alternatively, a low density fluid collection region, having a fluidoutlet, is provided at or in the region of the downstream end of thecore region, through which fluid, that is predominantly low densityfluid, is removed from the process. With such an arrangement, the fluidleaving the core region consists mostly of the low density fluid,perhaps with some entrained high density fluid and even a minor amountof entrained small solid particles. As this fluid stream has the lowdensity fluid in a high concentration, it is preferably kept separatefrom other fluid streams removed from the process. This stream may besubjected to a further separation in which entrained high densityliquids and entrained solids are removed.

In embodiments having a low density fluid collection region, a highdensity fluid collection region, having a second fluid outlet, isprovided downstream of the core region for the removal of high densityliquid. With such an arrangement, the fluid stream leaving through thesecond fluid outlet comprises as a major portion high density fluid,with some entrained low density fluid and/or solids. As this fluidstream has the high density fluid in a high concentration, it ispreferably kept separate from other fluid streams removed from theprocess. This stream may be subjected to a further separation in whichentrained high density liquids and entrained solids are removed.

In one embodiment, the solids and/or liquids separated from the fluidstreams leaving the first and/or second fluid outlets are recycled tothe separation region for further processing.

The or each fluid outlet may be in the form of a conventional pipe ortubular opening located centrally within the separation region. However,such openings when present in a concave rotating fluid stream tend togenerate a region of low pressure, the effect of which can be to disturbthe separation of solids and fluids already established in the annularregion. The or each fluid outlet is conveniently formed as an outletpipe, having a closed end and with fluid outlet apertures formed alongits length extending within the fluid collecting region. However, insuch a case, it is preferred that the combined area of the fluid outletapertures is greater than the cross-sectional area of the outlet pipe.In order to achieve a gradually distributed fluid take-off, theapertures are arranged over a length of the exit pipe. In addition, toreduce turbulence in the fluid collecting region, the apertures arepreferably arranged tangentially to the rotational flow of fluid in thecollecting region. This aids in reducing the shear applied to the fluid,which assists in maintaining the separation of fluid phases alreadyachieved in the preceding regions. This addresses a problem with knownseparators, the collectors of which can cause the phases to redisperseor, in particular inefficient systems, result in the phases emulsifying.

In embodiments having both a low density collecting region and a highdensity collecting region, and therefore first and second fluid outlets,one arrangement is to provide the outlets as openings in the sameconduit. The conduit may be arranged coaxially within the separationregion. The openings are preferably formed as hereinbefore described. Insuch an arrangement, the conduit extends from within both the lowdensity fluid collection region and the high density fluid collectionregion. The conduit may have a single outlet, resulting in the fluidstreams collected from the two collection regions being mixed.Preferably, the conduit is provided with a low density fluid outlet anda high density fluid outlet, thus maintaining the fluid separationalready achieved within the process.

In a preferred arrangement, a separate conduit is provided forcollection and transport of fluid from each of the low densitycollecting region and the high density collecting region. Each conduitextends into its respective region and is provided with one or moreopenings for the collection of fluid. The openings may be formed ashereinbefore described.

It is preferred to provide the conduit leading from the low densityfluid collecting region with a dip pipe or the like, in order tomaintain the appropriate fluid level within the assembly. Similarly, itis preferred to provide the conduit leading from the high density fluidcollecting region with a stand pipe, again to maintain a fluid levelwithin the assembly.

Downstream of the fluid collecting region or regions, the flowrate ofthe remaining fluid stream will be significantly lower. The stream atthis point contains a high proportion of solid particles entrained influid, predominantly high density fluid, with a minor portion of lowdensity fluid entrained therein. In order to prevent fluid vorticesextending into the regions downstream of the fluid collecting region, itis preferred to provide a means for controlling any vortices that mayform, such as a vortex breaker.

Downstream of the vortex breaker the stream is rich in solids andtypically is in the form of a solid slurry, in which solid particles aresuspended and entrained in the remaining fluid. Typically, this fluidwill be rich in the denser of the fluid phases entering the separationvessel. It is an advantage of the method of the present invention thatfurther separation of components from the stream may still be achieved.The remaining fluid can become more concentrated in the solid particlesby providing a solid concentrating region, in which the cross-sectionalarea of the fluid flowpath decreases in the direction of flow. Thereduction in cross-sectional area of the fluid flowpath may be achievedby providing a generally cylindrical vessel section with a central,longitudinally arranged cone. Alternatively, the vessel may be conicalin form.

It is important that the fluid stream in the solid concentrating regionis rotating, in order to prevent the solid particles in the fluid streamfrom settling out and forming a solid mass in the downstream region ofthe solid concentrating region. Such settling out renders the collectedsolids difficult to remove. However, it is also important that the flowstream is not rotating too fast, as this can reduce the efficiency ofthe solid concentrating region and require high volumes of fluid to beremoved in order to effect sufficient solids removal from the apparatus.In order to reduce the helical or rotational flow pattern of the fluidin the solid concentrating region, the region may be provided with oneor more baffles of appropriate shape and size to generate the optimumfluid flow pattern.

Preferably, the method of the present invention provides a final solidsseparation and removal region, the function of which is to separate thelarger solid particles, such as well debris, from the smaller solidparticles, such as sand and grit. In one embodiment, the smaller solidparticles are removed in the form of a slurry from the central region ofthe separation and removal region. This is conveniently achieved bymeans of an outlet pipe extending into the separation and removal regionand provided with a plurality of slurry outlet apertures. To minimisethe change in direction of the fluid, the apertures are preferablyarranged tangentially to the rotating fluid flow. Larger solid particlesremain in the outer region of the separation and removal region and maybe removed through one or more outlets. These outlets may convenientlybe one or more ports in the wall of the separation vessel. Again, to bein line with the fluid flow direction and minimise the risk of blockagesoccurring, the outlets are preferably oriented tangentially to therotating fluid flow.

In an alternative embodiment, the solids separation and removal regionis provided with an inner conduit having a plurality of slurry outletapertures forming a solids sieve. The fluid stream is caused to flowthrough the inner conduit. The rotation of the fluid stream causes thesolid particles to collect at the wall of the inner conduit, with thesmaller solid particles passing outwards through the solids sieve.Larger solid particles, such as well debris, remain within the conduitand are collected at the outlet of the conduit. The size distributionbetween solid particles retained within the inner conduit and thoseleaving through the sieve is determined by the diameter of the aperturesin the solids sieve.

The removal of the large diameter solid particles may be on a continuousbasis, or operated batch-wise, as the need arises. This will depend, inpart, upon the relative amounts of solid and fluid components present inthe feed stream, as well as the particle size distribution of the solidmaterial.

The streams containing solid particles, both large and small, may bepassed to a further separation process, for further concentration andremoval of the solids material. Fluid components removed during theseseparation processes may be recycled to the appropriate position in theseparation region.

In a further aspect, the present invention provides a separation systemfor a multiphase fluid containing a high density component and a lowdensity component comprising a separator having:

-   -   a separation region;    -   an inlet for the multiphase fluid to enter the separation        region;    -   means for imparting a rotational movement to the multiphase        fluid upon entry into the separation region, so as to form an        outer annular region of rotating fluid of predetermined        thickness and a core region;    -   in operation the thickness of the outer annular region being        such that the high density component is concentrated and        substantially contained within the outer annular region; and    -   the low density component is concentrated in the core region.

The separation system comprises a separation region. This is mostconveniently arranged as a generally cylindrical vessel. With the vesseldisposed substantially vertically, the inlet for the multiphase fluid isarranged in the upper portion of the vessel.

The system may comprise a single inlet for the multiphase fluid to enterthe separation region and a single inlet will suffice in manyapplications. However, if required, two or more fluid inlets may beprovided, for example disposed around the separation region to form afluid inlet region.

The system comprises a means for imparting a rotational motion to themultiphase fluid entering the separation region. The rotation is mostconveniently, and most preferably, by appropriate design of the fluidinlet. In particular, the rotational movement can be achieved byarranging the fluid inlet at a tangent to the separation region. Theinlet is preferably arranged at an acute angle to the longitudinal axisof the separation region. In this way, the inlet imparts a helical flowpattern to the fluid entering the system, thereby establishing theannular region of fluid along a portion of the length of the separationregion. The inlet may be at an angle of from 45° to 85°, more preferablyfrom 50° to 80°, to the longitudinal axis of the separation region. Theinlet may have any suitable cross-sectional shape. However, preferenceis given to an inlet having a rectangular cross-section. In this way, asmoother transition of the fluid flow from the inlet to the wall of theseparation region will be achieved. The dimensions of the inlet areselected to provide the flow pattern required in the outer annularregion of the separation region.

The separator system further comprises an arrangement of fluid outletsfor removing fluid from various points in the separation process. One ofa number of alternative arrangements for the fluid outlet may beemployed. In the first arrangement, both low density fluid and highdensity fluid is removed through the same outlet. This arrangement hasthe advantage of being simple. However, this arrangement tends to negatethe effects of the process in separating the fluid phases. Analternative arrangement is to provide two separate fluid outlets, placedwithin the separation region so as to selectively remove high densityfluid and low density fluid from their respective regions of highestconcentration. In this way, the fluid separation achieved by the processmay be retained. Details of the alternative arrangements for the fluidoutlet are described below.

In a first preferred embodiment, the system comprises a fluid outletdisposed within the separation region at a position that, in use,corresponds to a position downstream of the end of the core region. Theoutlet is preferably formed in a conduit extending into the separationregion. The outlet may simply be the open end of the conduit. However,this has been found to generate turbulence and shear in fluid flowingpast the opening, the tendency of which is to mix fluid phases that havealready begun to separate. Accordingly, the outlet is preferably in theform of a conduit having a closed end and provided with a plurality ofopenings in its end portion. With the conduit arranged longitudinallyand centrally within the separation region, the openings extend radiallyoutwards. In a preferred arrangement, the openings are arrangedtangentially to the flow of fluid occurring around the conduit duringuse. In this way, an enhanced fluid flow profile is created in the fluidflow pattern.

In an alternative arrangement, a first fluid outlet for low densityfluid is provided within the separation region at a positioncorresponding to the region at the downstream end of the core region. Inthis way, low density fluid that has collected within the core region isremoved in a stream that contains the low density fluid in a relativelyhigh concentration. The first fluid outlet is preferably formed in aconduit extending into the separation region, the details of which areas described above.

Preferably, in the case of a first fluid outlet disposed within the coreregion, a second fluid outlet is provided within the separation regionat a position corresponding to a position downstream of the core region,when the system is in use. In this way, fluid may be removed directlyfrom the region having the high density fluid in a high concentration.The second fluid outlet is preferably formed in a conduit extending intothe separation region, the details of which are as described above.

One arrangement comprises a single conduit extending within theseparation region and having openings disposed to provide the firstfluid outlet and the second fluid outlet. The conduit may have a singleoutlet for the fluid collected from the separation region. However, mostpreferably, the conduit has an outlet for each of the low density fluidand the high density fluid. In this way, the separation of the fluidphases achieved within the separation region is maintained.

In a preferred arrangement, each of the first fluid outlet and thesecond fluid outlet open into a separate conduit, for collection,transport and removal of the respective fluid fraction from theassembly. In this way, each fluid fraction may be lead away from theassembly. The conduit for the light fluid fraction preferably comprisesa dip pipe or the like, in order to maintain an appropriate fluid level.Similarly, the conduit for the heavier fluid fraction may comprise astand pipe.

In the region downstream of the core region and the fluid outlet oroutlets, the system preferably comprises a means for breaking and/orcontrolling vortices forming within the separation region.

Thereafter, in the downstream direction, the system preferably comprisesa region having the function of concentrating the most dense phases inthe stream being processed. This feature is particularly advantageouswhen the fluid stream being separated contains solid particles ofvarying sizes. The concentration region is provided by having thecross-sectional area of the separation region reduced. This may beachieved by providing the separator vessel with a conical or taperedinternal wall.

Alternatively, the necessary reduction in cross-sectional area may beachieved within a vessel of constant cross-section by provision of aconical or tapered element disposed within the separation region. Ifemployed, the conical or tapered element preferably comprises one ormore longitudinal bores or passages therethrough, in order to allowlower density fluid to vent from the region below the element to theregion above the element. The conical or tapered element will extend atthe angle required to give the appropriate change in cross-sectionalarea of the separator, in the direction of fluid flow. A typical angleof cone or taper is in the range of from 5 to 30° to the longitudinalaxis of the separator.

For the processing and separation of solid particles from the multiphasefluid stream, the system is preferably provided with a region forseparating solid particles from the fluid in the downstream portion ofthe separation region. In a first embodiment, the solid separatorcomprises a conduit extending coaxially within the separation regionhaving a plurality of openings in the wall of the conduit. The openingspreferably extend radially through the wall of the conduit, mostpreferably at a tangent to the surrounding fluid flow. In this way,fluid and smaller solid particles entrained in the fluid are caused topass through the openings into the conduit, from where the fluid andsolids may be recovered. The size of the openings may be selected toprovide the particle size distribution desired in the separated solids.In this way, the conduit acts as a form of filter, retaining the largerparticles within the separation region, from where they may be recoveredthrough a suitable outlet disposed in the wall of the separator region.

In an alternative arrangement, the wall of the separation region isprovided with a plurality of openings of appropriate size, the openingsconnecting the separation region with a solid entrapment zone disposedaround the separation region. In a preferred arrangement, the solidentrapment zone is an annular region extending around the separationzone and separated by the perforated wall of the separation zone. Underthe action of the rotating fluid, smaller particles will pass throughthe openings entrained in fluid, while larger solid particles will beretained within the separation region. Again, the size of the openingsmay be selected to provide the desired particle size distributionbetween the separated solids and act as a filter. The smaller particlesand fluid are removed from the solid entrapment zone through a suitableoutlet. Larger particles may be removed from the separation region,again through a suitable outlet.

The removal of large solid particles from the separation region may beoperated on a continuous basis. However, it may be preferable in somesituations to remove the larger solids intermittently or batchwise. Thismay be particularly useful when processing a fluid stream produced froma subsea well, in which may be entrained not only smaller solidparticles, such as sand and grit, but larger debris from damaged or worndownhole equipment and the like. The larger debris may be allowed toaccumulate in the downstream end of the separation region and removed atregular intervals, for example by means of a remotely operated vehicle(ROV).

A particular problem arises with existing arrangements of subseaprocessing assemblies. At the present time, it is the practice toinstall subsea separation assemblies to operate at reduced pressures.Examples of such assemblies are settling tanks and the like. Thisarrangement necessitates installing the separation units downstream ofprocess equipment such as chokes. A particular problem arises when thefluid being produced from the well contains solid material, such as sandand well debris. The process equipment located upstream of separatorunits is exposed to the flow of the fluids and entrained solids atsubstantially wellbore pressure. This results in a very rapid rate ofwear of the process equipment and frequent failures. Given the positionof such equipment in the process relative to the wellhead, it is anexpensive and time consuming task to repair or replace broken or faileditems. It is known to provide desanders immediately downstream of thewellhead, in order to remove sand from the produced fluids. Suchdesanders are cyclone separators, effective in removing sand particlesof a particle grade. However, such equipment is not effective in theremoval of larger solid particles or debris, which can be carried overand find their way into the downstream processing equipment causing thedamage mentioned before. Such debris may arise as a result of a failureor breakage of downhole equipment, resulting in large items of debrisbeing produced from the well. It is an advantage of the method andsystem of the present invention that this debris can be effectivelyremoved. Accordingly, the system of the present invention isparticularly suited to being situated immediately downstream of thewellhead, being able to operate as it does at wellhead pressures andremove or contain even the largest of debris produced by the well.

Accordingly, in a further aspect, the present invention provides asubsea processing assembly comprising:

-   -   a wellhead assembly through which fluids are produced from a        subterranean well;    -   a separator assembly having a fluid inlet connected to the        wellhead assembly for receiving the fluids produced from the        well, the separator assembly being operable at wellhead pressure        to remove well debris entrained in the fluids to produce a        solids-rich phase and a fluid phase, the separator assembly        comprising a fluid outlet for the fluid phase; and    -   a choke assembly having an inlet connected to the fluid outlet        of the separator assembly.

The separator assembly may be connected directly to the wellheadassembly or by means of a pipeline.

Similarly, the separation assembly of the present invention may beemployed on an offshore platform receiving production fluids directlyfrom either platform wellheads or a subsea wellhead through a riser orthe like upstream of the choke. Accordingly, in a further aspect, thepresent invention provides a platform processing assembly comprising:

-   -   a fluid receiving assembly for receiving fluids produced from a        subterranean well;    -   a separator assembly having a fluid inlet connected to the fluid        receiving assembly for receiving the fluids produced from the        well, the separator assembly being operable at wellhead pressure        to remove well debris entrained in the fluids to produce a        solids rich phase and a fluid phase, the separator assembly        comprising a fluid outlet for the fluid phase; and    -   a choke assembly having an inlet connected to the fluid outlet        of the separator assembly.

In this respect, the wellhead pressure at the fluid receiving assemblyof the platform will be the actual wellhead pressure less any pressuredrop due to the change in elevation or as a result of the passage of thefluid through the riser assembly.

As noted above, the method of the present invention is particularlysuitable for the separation of solid particles from a multiphase fluidstream containing gas and liquids. Accordingly, in a further aspect, thepresent invention provides a method for separating solid particles froma multiphase fluid stream, the fluid stream comprising a liquidcomponent and a gas component, the method comprising:

-   -   introducing the stream into a separation region;    -   imparting a rotational movement into the fluid;    -   forming an outer annular region of rotating fluid of        predetermined thickness; and    -   forming and maintaining a core of gas in an inner region;        wherein    -   liquid and solid particles entering the separation vessel are        directed to the outer annular region; and    -   the thickness of the outer annular region is such that the solid        particles are concentrated and substantially contained within        this region.

As has been hereinbefore described, a multiphase fluid stream isintroduced into a separation in such a way as to impart a helical,rotational flow pattern and to cause an outer annular region of heaviercomponents to form. Conventionally, equipment that relies upon theseparation of components using fluid rotation and the resultant forceshas introduced the combined stream into a suitable vessel or regionthrough one or more openings. In the development of the presentinvention, it has been found that this incoming stream can disrupt theflow patterns established within the separation region. It has beenfound that this disruption can be reduced, and the separation efficiencyincreased, by forcing the incoming fluid to flow along a circular orhelical path prior to entering the separation region. In this way, thecomponents within the fluid stream may be at least partially oriented ina corresponding manner to the orientation required in the separationregion.

Accordingly, in a further aspect, the present invention provides amethod of separating a multiphase fluid stream, the method comprisingintroducing the stream into a separation region in a manner to induce arotational flow pattern within the separation region, wherein, prior toits introduction into the separation region, the fluid stream is causedto flow along an arcuate flowpath, the fluid flowing along the arcuateflowpath in an orientation corresponding to the rotational flow patternwithin the separation region.

It has been found that under the appropriate flow regimes within thearcuate flowpath, the components within the fluid stream can be causedto separate according to their relative densities. This initialseparation may be used to enhance the separation required in theseparation region, provided the fluid inlet is oriented to introduce thefluid stream into the separation region such that the components of theincoming fluid are appropriately oriented. Thus, the arcuate flowpathand the fluid inlet are arranged such that the heavier components in thefluid stream are introduced radially outwards of the lighter components.

The arcuate flowpath may be circular or helical, as required by theparticular separation function to be achieved and the physical aspectsof the assembly being used.

The flow of fluid in the circular flowpath should be in a regime thatallows for separation of the components to begin. Accordingly, it ispreferred that the fluid stream in the arcuate flowpath is in a laminaror transitional flow regime, that is less than turbulent. The length andradius of curvature of the arcuate flowpath should be sufficient to atleast initialise separation of the heavier and lighter fractions priorto the stream entering the separation region.

In a further aspect, the present invention provides an apparatus forseparating a multiphase fluid stream, the apparatus comprising:

-   -   a separation region;    -   an inlet for introducing a fluid stream into the separation        region;    -   an arcuate conduit for conveying a fluid stream to the inlet;    -   wherein the arcuate conduit and the inlet are arranged to        introduce the fluid stream into the separation region in an        orientation corresponding to that of the fluid within the        separation region during operation.

The separation region may be that of any known or conventionalrotational separation device, such as a cyclone. In this respect, anexisting separation installation or assembly may be modified to operateaccording to this aspect of the invention by a suitable modification ofthe conduit conveying fluid to the inlet of the separation device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, having reference to the accompanying drawings, in which:

FIG. 1A is a schematic representation of the flow patterns of thecomponents of a multiphase fluid stream in the method of the presentinvention;

FIG. 1B is a schematic representation of alternative flow patterns ofthe components of a multiphase fluid stream in the method of the presentinvention;

FIG. 2 is a longitudinal cross-sectional view through a separator systemaccording to a first embodiment of the present invention;

FIG. 3A is a longitudinal cross-sectional view through a separatorsystem according to a second embodiment of the present invention;

FIG. 3B is a longitudinal cross-sectional view through a separatorsystem according to a third embodiment of the present invention;

FIG. 4 is a longitudinal cross-sectional view through a separator systemaccording to a fourth embodiment of the present invention;

FIG. 5 is a longitudinal cross-sectional view through a separatoraccording to a fifth embodiment of the present invention;

FIG. 6 is a partial cross-section view through a separator according toa sixth embodiment of the present invention;

FIG. 7 is a cross-sectional view through the inlet and separationregions of a separator according to a seventh embodiment of the presentinvention;

FIG. 8 is an enlarged cross-sectional view of the fluid inlet region ofthe separator of FIG. 7;

FIG. 9 is a representative view of a subsea processing assembly using aseparator assembly in accordance with various embodiments; and

FIG. 10 is a representative view of a platform processing assembly usinga separator in accordance with various embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

References to ‘upwards’ and ‘downwards’ as used herein refer to theassemblies with longitudinal axes in the vertical orientation as shownin the accompanying figures. It will however be understood thatnon-vertical orientations may also be applied and the aforementionedterms are to be construed accordingly.

Referring to FIG. 1A, there is shown a schematic representation of atypical fluid flow pattern within the separation region of the method ofthe present invention. A multiphase fluid 2 is introduced into avertically oriented separation region 4 through a single rectangulartangential inlet 6. The inlet 6 is at an angle to the longitudinal axisof the separation region 4, such that the incoming fluid 2 is directedagainst the wall of the separation region in a helically descending flowpattern 8. The general direction of fluid flow is indicated by the arrowin FIG. 1. The helical flow pattern establishes an outer annular region10 adjacent the wall of the separation region 4 within which is disposeda core region 12. The components of the multiphase fluid having a higherdensity are concentrated in the outer annular region 10, while the lowerdensity components migrate to and are collected in the core region 12.An interface 14 is established between the core region and the outerannular region. For example, when the multiphase fluid comprises solids,liquids and gases, the outer annular region 10 is essentially a rotatingwall of liquid, with entrained solids and some entrained gas bubbles. Incontrast, the core region 12 is composed of gas, with some entrainedliquid droplets. A rapid exchange of liquid and gas will occur acrossthe interface 14 between the core region and the annular region, as thefluid descends within the separation zone. As will be seen from FIG. 1,the interface 14 is convex in shape in the downstream direction of flow.

As shown in FIG. 1A, the helical flow pattern followed by the incomingfluid has a pitch such that the fluid completing its first revolutionpasses below the incoming fluid entering through the inlet 6. In thisway, the incoming fluid avoids colliding with the rotating fluid alreadywithin the separation region 4. This serves to quickly and efficientlyestablish the outer annular region at the thickness predetermined by thedimensions of the inlet 6.

As also shown in FIG. 1A, the inlet 6 is disposed some distance from theupper end of the separation region 4. In this way, high density fluidand entrained solids splashing from the outer annular region 10 arecaused to drop back into the fluid stream and become entrained in thefluid circulating in the outer annular region 10. In this way, theerosion of the top of the vessel wall containing the separation region 4is minimised.

FIG. 1B shows a similar representation of fluid flow patterns to that ofFIG. 1A. Features of FIG. 1B shared with FIG. 1A are indicated using thesame reference numerals. In FIG. 1B, the multiphase fluid is dividedinto two streams 2 and 3 and is caused to enter the separation region 4through two opposed rectangular inlets 6 and 7. Each inlet 6 and 7 isarranged tangentially to the wall of the separation region 4. The inlets6 and 7 are angled such that the incoming fluid from the inlet 7 flowsbelow and under the fluid stream 2 entering through the inlet 6, inorder to avoid collision between the fluid streams 2 and 3. It will beappreciated that three or more inlets may be arranged, applying asimilar approach to that of FIG. 1B, such that multiple fluid streamsare introduced into the separation region 4 with minimal or no collisionbetween the incoming fluid streams.

Referring to FIG. 2, there is shown, in vertical cross-section, aseparator system according to one embodiment of the present invention.The separator system, generally indicated as 102, comprises a generallycylindrical, vertically oriented separator 104, having a cap 106 mountedon the upper end of the separator by means of a flange 108 and bolts110. A tapered solid concentrator 112 is secured to the lower end of theseparator 104 by means of flanges 114 and bolts 116. As shown in FIG. 2,the solid concentrator 112 has a conical inner portion. The angle of theconical section will be determined by the properties of the fluid beingprocessed. Cone angles in the range of from 5 to 20° from thelongitudinal axis of the separator are typical for the separation of afluid stream comprising crude oil, water and solids.

The separator 104 has a generally cylindrical bore running therethrough,extending in its upper portion into the cap 106 and, in its lowerportion, partially into the solid concentrator 112. The remainingportion of the solid concentrator 112 is formed with a tapered bore,decreasing in cross-sectional area in the downwards direction andopening into a slurry container 118 mounted to the lower end of thesolid concentrator 112 by means of flanges 120 and bolts 122. Thecombined bores of the cap 106, separator 104, solid concentrator 112 andslurry container 118 form a separation region. The components formingthe separation region are of sufficiently wear resistant material toavoid excessive erosion of the walls or may be lined or sleeved in suchmaterial.

The cap 106 is provided with a feed pipe 124 communicating with arectangular inlet 126 in the cap. The feed pipe 124 and the rectangularinlet 126 are arranged at an acute angle to the longitudinal axis of theseparator and tangentially to the wall of the cap 106. The angle of thefeed pipe 124 and the inlet 126 will be determined by the properties ofthe fluid stream and the separation duty to be performed. A typicalangle for the feed pipe 124 and inlet 126 is from 5 to 20°, measuredfrom a line perpendicular to the longitudinal axis of the separator 104.

The dimensions of the inlet opening will also be determined by the fluidstream and the separation duty. For a fluid stream comprising a viscouscrude oil, water and sand particles, the inlet 126 will be sized toprovide a fluid inlet velocity of from 1 to 5 m/s. The relativedimensions of the inlet opening, that is its aspect ratio, aredetermined by the fluid properties and the separation and the requiredflow pattern within the separator 106. In particular, the aspect ratioof the inlet 126 will affect the thickness of the outer annular region,in which the heavier components will concentrate.

A fluid outlet pipe 128 extends co-axially from the closed end of thecap 106 down into the solid concentrator 112. At its upper end, thefluid outlet pipe 128 communicates with an outlet 130 in the cap 106, towhich is connected to the inlet of a production fluid choke 132 ofconventional design, for example a plug-and-cage choke. The outlet ofthe production fluid choke 132 is connected to a fluid line 134 leadingto downstream fluid processing equipment. The lower end of the fluidoutlet pipe 128 coincides generally with the junction between theseparator 104 and the solid concentrator 112 and is closed. The lowerend portion of the fluid outlet pipe 128 comprises a plurality of fluidports 136 extending in a radially outwards direction at a tangent to theouter surface of the fluid outlet pipe 128.

A vortex control assembly 138 is provided and mounted on the lower endof the fluid outlet pipe 128, and comprises a fluid guide 140 in theform of an inverted cone to provide a vortex flow foil. The fluid guide140 acts to disseminate the upwards flowing vortex. This induces acurved flow path allowing fluid to circulate and enter the tangentialports 136 without fluid suction arising. The fluid guide 140 also forcesthe heavier components outwards, thus preventing a premature widening ofthe outer annular region and pushing the interface 125 radiallyoutwards. This further enhances separation of the components of thefluid stream.

The slurry container 118 comprises a generally cylindrical bore oflarger internal diameter than the diameter of the lower end of thetapered bore in the solid concentrator 112. A perforated cage 142 havingan internal diameter corresponding to that of the lower end of thetapered bore in the solid concentrator extends co-axially through theslurry container 118 and forms an annular region 144 in the slurrycontainer. The perforations 146 in the cage 142 are arrangedtangentially and connect the bore of the cage 142 with the annularregion 144. A tangentially arranged fluid outlet is provided in thelower portion of the annular region 144 of the slurry container 118 andconnected to the inlet of a slurry choke 148 of conventional design, forexample a stem-and-orifice choke. The outlet of the slurry choke 148 isconnected to a slurry line 150 leading to the downstream slurryprocessing equipment.

The perforated cage 142 is connected at its lower end to an outlet 152in the slurry container 118, which is in turn connected by a hydraulicor manual ROV valve 154 to a debris line 156, through which debriscollected in the system may be collected and removed.

In operation, a multiphase fluid stream comprising gas, one or moreliquid phases, and solid particles ranging from sand to well debris isfed through the feed pipe 124 to the rectangular inlet 126 in the cap106. Such a multiphase fluid is typical of the stream produced from asubterranean well. The fluid stream enters the separation region in thecap 106 and flows in a helical pattern downwards within the cap 106 andseparator. The liquid and solid phases present in the fluid stream aresubstantially confined to flow in an annular region adjacent the wall ofthe cap 106 and separator. A core region consisting essentially of gasis maintained within the separation region, the interface between thecore region and the annular region being shown by the dotted line 125 inFIG. 2. As rotating liquid stream descends within the separator 104,entrained gas is caused to pass into the gas core. Liquid droplets andany solid particles that may be entrained within the gas core are causedto move in the opposite direction and enter the liquid annular region.

As the liquid stream in the annular region descends, it loses angularmomentum, resulting in the annular region becoming larger incross-section as the separator 104 is descended and the core regionsmaller in cross-section, until the liquid in the annular region extendsacross the bore of the separator. This action forms the convex shape ofthe interface shown in FIG. 2 and in more detail in FIGS. 1A and 1B. Thecontinued rotation of the liquid causes the more dense phases, includingthe solid particles, to collect at the wall of the separator, while theless dense liquid and gas phases will move towards the central axis ofthe separator 104.

Clean fluid is removed from within the separator 104 through the fluidports 136 in the lower portion of the fluid outlet conduit 128. Thisfluid will comprise both the less dense liquids from the annular region,as well as gas displaced downwards from the core region. This combinedfluid stream exits the conduit 128 through the outlet 130 in the cap 106and passes via the production fluid choke 132 to the fluid line 134 forfurther processing.

The rotational action of the liquids remaining in the separator cancreate a vortex, which will have the effect of causing fluid flow to bedrawn up from the rotating slurry fluid and reducing the separation ofthe phases, in particular the separation and gradation of the solidparticles. The vortex control assembly 138, in particular the vortexflow foil 140, on the lower end of the fluid outlet conduit 128 preventsthe vortex from drawing slurry from the slurry separation region andpassing upwards into the region around the ports 136 in the fluid outletconduit 128. The vortex flow foil 140 also acts to push the downwardfluid flow outwards towards the wall of the separation region, furtherenhancing solids separation.

From the separator 104, fluid passes into the solid concentrator 112.The cross-sectional area of the flowpath of the fluid is reduced alongthe length of the solid concentrator 112 by the combined effects of thetapered bore of the concentrator 112 and the conical fluid guide 140,causing the fluid to become concentrated in the solid particles and forma slurry.

The slurry passes to the central perforated cage 142 of the slurrycontainer 118, where the vortex acts to rotationally agitate the fluidand entrained solids. Fluid and smaller solid particles pass outwardsthrough the perforations 146 in the perforated cage 142 as the slurrydescends within the slurry container and are removed from the annularregion 144 through the outlet to pass to the slurry choke 148. Soliddebris remains within the perforated cage 142 and leaves the slurrycontainer 118 through the lower outlet 152. Depending upon the amount ofdebris in the stream being processed, the hydraulic or manual ROV valve154 may be left open, to provide a continuous flow of debris, or may beopened intermittently, for example by a remote operated vehicle (ROV),to empty the bore of the slurry container when sufficient debris hasbeen collected. The ROV may be provided with a receptacle for receivingthe solid debris for removal.

Referring to FIG. 3A, there is shown an alternative embodiment of theseparator system of the present invention. The components of the systemof FIG. 3A corresponding to those of the system of FIG. 2 are indicatedusing the same reference numerals. The general manner of operation ofthe system of FIG. 3A is largely the same as that of FIG. 2.Accordingly, to avoid repetition, only the differences in constructionand detailed operation between the systems of FIGS. 2 and 3A will bedescribed.

The system of FIG. 3A comprises a slurry container 118 mounted directlyon the lower end of the separator 104, the separation region beingformed by the combined generally cylindrical bores of the cap 106, theseparator 104 and the slurry cage 118. The lower end of the fluid outletconduit 128 is closed with a conical cap 140. A slurry collection cage172 extends co-axially upwards within the slurry container 118. Aplurality of tangential perforations 176 are formed in the slurrycollection pipe 172. The lower end of the slurry collection cage 172connects with a slurry outlet 178 in the lower end of the slurrycontainer 118, which in turn connects with a slurry choke 148 and slurryline 150. A tangential debris outlet port 180 is disposed in the wall ofthe slurry container 118 adjacent its end, which is connected to ahydraulic or manual ROV valve 154 and a debris line 156.

In operation, the separator system of FIG. 3A provides an alternativemeans of solid concentration and solid collection. The cross-sectionalarea of the downstream end of the bore within the separator 104available for the flow of fluid is reduced by means of the conical cap174 on the slurry collection cage 172, causing an annular concentrationof solids within the fluid. This causes the concentration of the solidparticles within the fluid phase as the stream descends from theseparator 104 to the slurry container 118 forming a region of relativelystill fluid, allowing solid particles to settle and form a concentratedslurry. Fluid is removed from the slurry container 118 by passingthrough the tangential perforations 176 in the slurry collection pipe172.

Referring to FIG. 3B, there is shown an alternative arrangement to thatof FIG. 3A, but employing the same principle for the collection andremoval of solid particles using a slurry collection cage 172. Theassembly of FIG. 3B employs a generally cylindrical separator 104, theslurry container having a diameter substantially the same as that of theseparator 104. To provide a constriction in the downward fluid flowpath,the slurry collection cage 172 has its upper end closed by a conical cap174. A vent port 175 extends vertically through the conical cap 174. Thevent port 175 in the conical cap 174 provides an outlet for fluiddisplaced from the slurry container 118.

In the slurry container 118, fluid and solid particles are caused to tryand concentrate on the separator axis and to flow through the tangentialslots 176 and enter the slurry collection cage 172, from where they aredischarged through the slurry outlet 178, via the slurry choke 148 intothe slurry line 150. Debris and very coarse solids in a fluid slurry areremoved from the slurry container through the debris outlet 180 and viathe hydraulic or manual ROV valve 154 into the debris line 156.

The arrangements of FIGS. 3A and 3B offers advantages over that of FIG.2 in the case that the fluid stream contains solid particles that areeasily entrained. The arrangements of FIGS. 3A and 3B provide arelatively large volume of substantially stationary fluid in the slurrycontainer 118, surrounding the slurry collection cage. This arrangementprovides an increased slurry residence time in the slurry container 118of FIGS. 3A and 3B, compared with that of FIG. 2. Easily entrainedparticles are thus allowed a greater time for settling in the slurrycontainer 118, in turn increasing the separation efficiency of theassembly.

Referring to FIG. 4, there is shown a further embodiment of theseparator system of the present invention. The system of FIG. 4comprises a cap, a separator and a slurry container substantially asdescribed hereinbefore and as shown in FIG. 3 and operates in the samegeneral manner as the system shown in FIG. 3. Accordingly, componentscommon to the systems of both FIGS. 3 and 4 are indicated using the samereference numerals. However, the arrangement shown in FIG. 4 employs analternative fluid collection regime, the components and operation ofwhich will now be described in more detail.

The separation system of FIG. 4 employs a light fluid outlet conduit 200and a heavier fluid outlet conduit 205. The heavier fluid outlet conduit205 is arranged co-axially within the separation region and extendsthrough the slurry container 118. The heavier fluid outlet conduit 205extends co-axially at its lower end portion within the slurry collectioncage 172, forming an annular chamber for the collection of slurrythrough the slots.

The light fluid outlet conduit 200 has a vortex arrestor assembly 201 atthe gas liquid level 203 to hinder the formation of a vortex and preventheavier fluid being drawn into the light fluid outlet conduit 200. Thelight fluid outlet conduit 200 is provided with a plurality oftangential fluid ports 211 disposed so as to open into the core regionabove the vortex arrestor 201. As shown in FIG. 4, the interface betweenthe core region and annular region within the separator 104 is indicatedby the dotted line 125. It is preferred that the light fluid outletconduit 200 is of a length such that, in operation, the lower ordownstream end of the core region intersects the light fluid outletconduit 200 at or close to its lowermost end, above the vortex arrestorassembly 201.

The fluid outlet 130 in the cap 106 connects with a liquid trap 204mounted to the upper end of the cap 106 by a flange and bolts. Theliquid trap 204 comprises a central chamber 206 and a fluid feed pipe208 extending co-axially upwards within the chamber 206 from the fluidoutlet 130 in the cap 106. The fluid feed pipe 208 has its upper endsealed with a domed cap 210 and a plurality of tangential fluid ports212 opening into the chamber 206. A tangentially arranged fluid outlet214 is disposed in the upper portion of the wall of the liquid trap 204,which connects the chamber 206 with a fluid line 216. A plurality ofdrain ports 218 extend from the lower end of the chamber 206 in theliquid trap 204 to corresponding ports 220 formed in the end of the cap106 opening into the separation region to enable liquid to berecirculated as shown in FIGS. 1A and 1B.

The heavier fluid outlet conduit 205 is capped at its upper end by adome cap 209 and is provided in its upper portion with a plurality oftangential fluid ports 213. The heavier fluid outlet conduit 205connects with a fluid outlet port 222 in the slurry container 118, whichin turn is connected to a fluid line 224. The fluid line is shaped toform a weir 225 having a level corresponding to that of the downstreamend of the core region, as indicated in FIG. 4.

A cone 226 extends around the heavier fluid outlet conduit 205 in aregion below the tangential fluid ports 213. The cone 226 is providewith an annular passage 228 adjacent the outer surface of the heavierfluid outlet conduit 205, to provide a passage for fluid to pass upwardsand out of the slurry container 118 for collection.

In operation, fluid is removed from the separator in two ways. First,the least dense fluid collected in the core region, most typically gas,passes through the ports 211 in the fluid outlet conduit 200 and flowsin an upstream direction through the cap 106 and into the fluid feedpipe 208 in the liquid trap 204. The fluid leaves the fluid feed pipe208 through the tangential ports 212, imparting a rotational flowpattern to the fluid in the chamber 206. Dense phases, such as liquidand any entrained solid particles, move to the wall of the chamber 106and flow downwards, returning to the separation region within the cap106 through the drain ports 218 and 220 and due to the angled tangentialinlet flow, is entrapped and removed in the main flow stream, as shownin FIGS. 1A and 1B. The remaining fluid, typically gas, leaves thechamber 206 through the outlet 214 and passes into the fluid line 216.The fluid line 216 is shaped so as to form a gas weir 217.

Denser fluid leaves the separator downstream of the core region bypassing through the ports 213 in the heavier fluid outlet conduit 205,and flows in a downstream direction within the conduit 205 through theslurry cage and into the fluid line 224 via the fluid outlet 222 in theslurry container 118.

A further embodiment of the present invention is shown in FIG. 5. Thesystem of FIG. 5 comprises a cap, a separator and a slurry cagesubstantially as described hereinbefore and as shown in FIG. 3 andoperates in the same general manner as the system shown in FIG. 3.Accordingly, components common to the systems of both FIGS. 3 and 5 areindicated using the same reference numerals. However, the arrangementshown in FIG. 5 employs a further alternative fluid collection regime,the components and operation of which will now be described in moredetail.

Referring to FIG. 5, a fluid collection assembly 300 extends co-axiallywithin the separation region from the end of the cap 106 to the regionof the junction between the separator 104 and the slurry container 118.The fluid collection assembly 300 comprises inner and outer conduits 302and 304 arranged concentrically so as to form an annular channel 306between the two. The lower end of the outer conduit 304 is closed. Inoperation, the lower end of the outer conduit 304 will lie at thegas/liquid level 203. A plurality of fluid ports 310 are provided in theouter conduit 304 and correspond in form, arrangement and function tothe ports 211 shown in FIG. 4 and described above.

The inner conduit 302 extends within the outer conduit 304 and projectsfrom the lower end of the inner conduit 302, such that its lower end isdisposed below the core region, when the separator system is inoperation. The inner conduit 302 is provided in its lower end portionwith fluid ports 213 of the type described and shown in FIG. 4. A vortexarrestor assembly 308 is disposed on the lower end of the inner conduit302.

The cap 106 is provided with an annular liquid knockout chamber 312connected to the annulus 306 in the fluid collection assembly 300. Theinner conduit 302 extends through the annular liquid knockout chamber312 to a first fluid outlet 314 in the end of the cap 106, which in turnconnects to a fluid line 316. The cap comprises a second fluid outlet318 in the wall of the liquid knockout chamber 312, which is connectedto a fluid line 320.

The fluid line 316 extends from the cap 106 to a level below theeffective end of the outer conduit 304 of the fluid outlet assembly 300.In this way, the flow of fluid through the fluid line 316 provides asiphon to aid fluid removal from the separator.

The liquid knockout chamber 312 is connected to the separation regionwithin the cap 106 by means of a plurality of fluid return ports 326.

In operation, fluid enters the fluid outlet assembly 300 through theports 310 in the outer conduit 304 from both the core region and theouter annular region of the separator 104. The low density fluid fromthe core region passes upwards through the annulus 306 into the liquidknockout chamber 312 in the cap 106. In the scenario describedhereinbefore, this will consist mostly of gas. Higher densitycomponents, such as liquid, are removed from the low density fluid inthe liquid knockout chamber 312 and return to the separation region inthe cap by way of the return ports 326. The low density fluid leaves thecap through the second fluid outlet 318 and enters the fluid line 320,which is shaped so as to form a gas weir 217.

Fluid from the annular region of the separator is drawn into the innerconduit 302 and passes upwards through the cap 106 and leaves via thefirst fluid outlet port 314. In the scenario outlined above, this fluidwill consist essentially of liquid, with some entrained gas. Theremaining fluid in the fluid line 316 is passed to downstream equipmentfor further processing.

It has been found that an assembly according to the present invention asshown in the accompanying figures can separate a combined stream ofcrude oil, water and solid debris at flowrates up to 25,000 BPD at veryhigh efficiency. In addition, the assembly can operate with a very highturn down ratio, that is a given assembly can operate over a wide rangeof fluid flowrates. For example, the assemblies shown in theaccompanying figures can operate as low as 5,000 BPD to separate acombined crude oil, water and solids stream. Below these flowrates,separation of the components due to fluid rotation diminishes and theassembly will operate under gravity separation principles. Accordingly,separation at flowrates from zero upwards may be achieved.

Referring to FIG. 6, there is shown a separation assembly according tothe present invention incorporating an inlet assembly according to thefinal aspect of the present invention. Features of FIG. 6 common toother embodiments of the present invention are indicated using the samereference numerals. The inlet assembly, generally indicated as 500,comprises a feed conduit 502 through which a fluid to be separated isfed to the assembly. The inlet assembly 500 further comprises an arcuateconduit 504 through which fluid may be conveyed to feed pipe 124 and theinlet 126.

As shown in FIG. 6, the arcuate conduit 504 extends in a helicalpattern, with the fluid completing two complete turns between the feedconduit 502 and the inlet 126. It will be appreciated that the arcuateconduit 504 may be arranged in a different configuration, for example ahelical pattern with greater or fewer turns, as may be required. Asshown in FIG. 6, the arcuate conduit 504 is a helical pipe. It will beappreciated that an arcuate flowpath for the fluid may be obtained byproviding an arcuate conduit of a different configuration.

In operation, a fluid stream to be separated in the separation assemblyfirst flows along the arcuate conduit 504, within which the componentsof the fluid stream begin to separate according to their relativedensities. Thus, the heavier components, for example liquids, such aswater, and solids, will tend towards the radially outer regions of theconduit. In contrast, the lighter components, such as light liquids, forexample oil, and gases, will tend to the radially inner portion of theconduit. The conduit 504 and the inlet 126 are arranged such that fluidstream is properly oriented with the flow patterns prevailing within theseparation region. Thus, the heavier components enter the separator 104in a radially outer position and the lighter components enter theseparator 104 in a radially inner position. It will thus be appreciatedthat the separation initialised within the arcuate conduit 504supplements the separation taking place within the separator 104.

Referring to FIGS. 7 and 8, there is shown a separator according to afurther embodiment of the present invention. The separator has the samegeneral configuration as that of FIG. 4 and the components common to theembodiment of FIGS. 7 and 8 and FIG. 4 are identified using the samereference numerals. Reference to the foregoing detailed description ismade with respect to these common features and components. The followingdescription relates to the features particular to the separator of FIGS.7 and 8. For clarity, FIGS. 7 and 8 show only the internal components ofthe separation system, with the external components, such as the inletassembly, and fluid and solid outlet assemblies being omitted.

The separation assembly of FIG. 7 has a tangential inlet assembly 602,as hereinbefore described, through which a fluid stream is introducedinto the upstream region of the separation region. The inlet assembly602 has an inlet opening 604 that opens into the separation region somedistance from the upstream end of the separation region. A wall assembly606 is provided having a helically extending guide surface 608 disposedsuch that incoming fluid contacts the guide surface 608 and is caused toflow in a helical pattern downwards within the separation region. Theinlet region is shown in more detail in FIG. 8, with the wall assembly606 and the guide surface 608 being shown in greater detail. Thepresence of the wall assembly 606 allows the inlet assembly 602 to beangled more closely to the perpendicular to the longitudinal axis of theseparation region, while still allowing the incoming fluid stream todevelop the required helical flow pattern within the separation region.Thus, the angle α of the inlet conduit as shown in FIG. 8 isapproximately 5o. A further effect of the wall assembly and the use ofthe guide surface 608 is to ensure that the incoming fluid is not causedto contact and impact the fluid already rotating within the separationregion. This in turn reduces the shear to which the fluid is subjected,improving the separation efficiency of the system.

The separator of FIG. 7 further comprises a slurry container 618 andslurry collection assembly, generally as hereinbefore described, inparticular as shown in FIG. 3B. The separator comprises a slurry cage620. To provide a constriction in the downward fluid flowpath, theslurry collection cage 620 has its upper end closed by a conical cap622. A vent port 624 extends vertically through the conical cap 622. Thevent port 624 in the conical cap 624 provides an outlet for fluiddisplaced from the slurry container 618.

In the slurry container 618, fluid and solid particles are caused to tryand concentrate on the separator axis and to flow through the tangentialslots 630 and enter the slurry collection cage 620, from where they aredischarged through the slurry outlet 632. Debris and very coarse solidsin a fluid slurry do not enter the slurry cage 620 and are removed fromthe slurry container through the debris outlet 634. The slurrycollection cage 620 of the separator assembly of FIG. 7 is extended inlength, compared with that shown in FIG. 3B. To reduce the rotation ofthe fluid stream in the slurry container 618 and enhance the separationof solids and fluids, the slurry cage 620 is provided with a pluralityof baffles 640 extending radially outwards across the slurry container.The baffles 640 are of such a size, number and pitch as to ensure thatthe fluid stream is sufficiently slowed but still moving enough toprovide for easy purging of the solid slurry through the debris outlet634.

To enhance the separation of solids from the fluid phases, the innerwall of the slurry container 618 is provided with a conical portion 650adjacent the debris outlet 634, in order to reduce the cross-sectionalarea of the slurry container in the downstream direction.

Referring to FIG. 9, there is shown a subsea processing assemblescomprising a wellhead assembly 902 through which fluids are producedfrom a subterranean well. The subsea processing assembly 900 alsoincludes a separator assembly 904 having a fluid inlet connected to thewellhead assembly for receiving the fluids produced from the well, theseparator assembly being operable at wellhead pressure to remove welldebris entrained in the fluids to produce a solids-rich phase and afluid phase, the separator assembly 904 comprising a fluid outlet forthe fluid phase. The subsea processing assembly also comprises a chokeassembly 906 (similar to the choke 132 discussed above) having an inletconnected to the fluid outlet of the separator assembly 904.

Referring to FIG. 10, the separation assembly of the present inventionmay be employed on an offshore platform 1000 receiving production fluidsdirectly from either platform wellheads or a subsea wellhead 1002through a riser 1004 or the like upstream of the choke (similar to thechoke 132 discussed above). Accordingly, in a further aspect, thepresent invention provides a platform processing assembly 1006comprising assembly 1008 for receiving fluids produced from asubterranean well. The platform processing assembles further comprises aseparator assembly 1010 having a fluid inlet connected to the fluidreceiving assembly 1008 for receiving the fluids produced from the well,the separator assembly 1010 being operable at wellhead pressure toremove well debris entrained in the fluids to produce a solids richphase and a fluid phase, the separator assembly 1010 comprising a fluidoutlet for the fluid phase. The platform processing assembly 1006further comprises a choke assembly 1012 having an inlet connected to thefluid outlet of the separator assembly 1010.

The invention claimed is:
 1. A method for separating a multiphase fluid,the fluid comprising a relatively high density fluid and/or solidcomponent and a relatively low density fluid and/or solid component, themethod comprising: introducing the multiphase fluid into a separationregion at an inlet; imparting a rotational movement to the multiphasefluid to separate at least some of the high density component from thelow density component with high density component moving to an outerannular region and the low density component moving to in an innerregion; forming an interface between the regions, the interfaceincluding a profile and forming a transition zone for the componentsbeing separated; and removing low density component from the innerregion through a restricted outlet while maintaining a steady-statecondition of the inner region.
 2. The method according to claim 1,wherein the phases of the multiphase fluid comprise at least twodifferent phases of the following: a liquid phase, a gaseous phase, anda solid phase.
 3. The method according to claim 1, wherein themultiphase fluid is produced from a subterranean oil well.
 4. The methodaccording to claim 3, wherein the multiphase fluid comprises solidformation materials and/or solid debris.
 5. The method according toclaim 1, wherein introducing multiphase fluid into the separation regioncomprises: introducing the multiphase fluid tangentially into theseparation region, thereby causing the fluid in the outer annular regionto rotate within the separation region; and introducing the multiphasefluid at an acute angle to the longitudinal axis of the separationregion, such as to avoid the fluid rotating in the outer annular regionfrom impacting the fluid entering the separation region.
 6. The methodaccording to claim 1, wherein introducing the multiphase fluid into theseparation region comprises contacting the multiphase fluid with a guidesurface, the guide surface inducing a helical flow pattern in themultiphase fluid within the separation region.
 7. The method accordingto claim 1, further comprising removing high density fluid and lowdensity fluid from a fluid collecting region established in theseparation region at a downstream end of the inner region.
 8. The methodaccording to claim 1, further comprising: establishing a low densityfluid collecting region in the region of a downstream end of the innerregion in the separation region; wherein the low density component is afluid and removing low density fluid from the low density fluidcollecting region; establishing a high density fluid collecting regionwithin the separation region downstream of the inner region; and whereinthe high density component is a fluid and removing high density fluidfrom the high density fluid collecting region.
 9. The method accordingto claim 8, further comprising removing high density fluid and lowdensity fluid from their respective fluid collecting regions by means ofseparate conduits, each conduit comprising a plurality of fluidapertures.
 10. The method according to claim 1, further comprisingproviding a solid concentrating region in the separation regiondownstream of the outer annular and inner regions, the solidconcentrating region comprising a flowpath that decreases incross-sectional area in the direction of flow.
 11. The method accordingto claim 1, further comprising: the high density component of themultiphase fluid including a solid phase and a fluid phase; providing asolids separation and removal region in the separation region downstreamof the inner and outer annular regions; moving high density componentdownstream from the outer annual region into the solids separation andremoval region; causing smaller solid particles to leave the solidsseparation and removal region through a central fluid outlet arrangedcentrally within the region; and removing larger diameter solidparticles from the solids separation and removal region through a solidsoutlet.
 12. The method according to claim 11, wherein the central fluidoutlet comprises an inner conduit through which the fluid phase iscaused to flow, the inner conduit comprising a plurality of outletapertures.
 13. The method according to claim 1, further comprising: thelow density and high density components each including a fluid phase;passing low density component fluid and high density component fluidremoved from the inner region to a fluid separation zone; separating thehigh density component fluid from the low density component fluid in thefluid separation zone; and returning the high density component fluid tothe outer annular region in the separation region.
 14. The methodaccording to claim 1, wherein removing low density component furthercomprises removing low density component from the inner regiondownstream of the inlet and reintroducing a portion of the low densitycomponent so removed into the inner region adjacent the inlet of themultiphase fluid.
 15. The method of claim 1, further comprising: thecomponents of the multiphase fluid comprising a liquid phase, a gaseousphase, and a solid phase; wherein forming an interface between theregions comprises forming a core of gas in the inner region as the lowdensity component; directing liquid and solid particles entering theseparation vessel to the outer annular region as the high densitycomponent; and wherein the thickness of the outer annular region is suchthat the solid particles are concentrated and substantially containedwithin the outer annular region.
 16. The method of claim 1, furthercomprising causing the multiphase fluid to flow along an arcuateflowpath prior to introduction into the separation region, the fluidflowing along the arcuate flowpath in an orientation corresponding tothe rotational movement within the separation region.
 17. The methodaccording to claim 16, wherein the arcuate flowpath is helical.
 18. Themethod according to claim 16, wherein the fluid in the arcuate flowpathis flowing in a laminar or transitional flow regime.
 19. A separationsystem for a multiphase fluid comprising a high density fluid and/orsolid component and a low density fluid and/or solid component, thesystem comprising a separator comprising: a separation region comprisingan inlet for the multiphase fluid to enter the separation region; meansfor imparting a rotational movement to the multiphase fluid upon entryinto the separation region, so as separate the high density componentfrom the low density component with the high density component moving toan outer annular region and the low density component moving to in aninner region; in operation, the thickness of the outer annular regionbeing such that the high density component is concentrated andsubstantially contained within the outer annular region and the lowdensity component is concentrated in the inner region so as to form aninterface between the regions, the interface including a profile andforming a transition zone for the components being separated; and anoutlet for the low density component comprising apertures such that thelow density component is removable from the separator through the outletwhile maintaining a steady-state condition of the inner region.
 20. Theseparation system according to claim 19, wherein the means for impartinga rotational movement to the multiphase fluid comprises the inlet beingtangential and at an acute angle to a longitudinal axis of theseparation region.
 21. The separation system according to claim 19,wherein the separation region further comprises a guide adjacent theinlet, the guide comprising a helically extending guide surface disposedto be impacted by the multiphase fluid entering the separation regionthrough the inlet.
 22. The separation system according to claim 19,further comprising the outlet for the low density component beingdisposed in a portion of the separation region at the downstream end ofthe inner region formed during operation.
 23. The separation systemaccording to claim 22, wherein the outlet for the low density componentis formed in a conduit extending into the separation region comprisingthe apertures.
 24. The separation system according to claim 23, furthercomprising an outlet for the high density component disposed in aportion of the separation region at the downstream end of the innerregion formed during operation.
 25. The separation system according toclaim 24, wherein the outlet for the high density component is formed ina conduit extending into the separation region and comprising apertures.26. The separation system according to claim 25, wherein the outlets forthe low density and high density components open into the same conduit.27. The separation system according to claim 26, further comprising asolids concentration region within the separation region, the solidsconcentration region comprising a cross-sectional area less than thecross-sectional area of the separation region adjacent the inlet. 28.The separation system according to claim 19, further comprising a vortexcontroller within the separation region downstream of the inner regionformed during use.
 29. The separation system according to claim 19,further comprising a means for separating solids from fluid within theseparation region.
 30. The separation system according to claim 29,wherein the means for separating solids comprises a perforated conduitextending within the separation region, the perforated conduit having aplurality of radially extending openings.
 31. The separation systemaccording to claim 29, wherein the means for separating solids comprisesa solid entrapment zone disposed around the separation region andseparated from the separation region by a wall having a plurality ofradially extending openings.
 32. The separation system of claim 19wherein: the separation system is used in a subsea processing assemblycomprising: a wellhead assembly through which fluids are produced from asubterranean well, the separation system inlet being connected to thewellhead assembly for receiving the fluids produced from the well; and achoke assembly comprising an inlet connected to the outlet for the lowdensity component; and the fluids from the well are the multiphase fluidand the separation system is operable at wellhead pressure to removewell debris entrained in the fluids to produce a solids phase and afluid phase, the outlet for the low density component used for removingthe fluid phase.
 33. The separation system of claim 19, wherein: theseparation system is used in a platform processing assembly comprising:a fluid receiving assembly for receiving fluids produced from asubterranean well, the separation system inlet being connected to thefluid receiving assembly for receiving the fluids produced from the wellas the multiphase fluid; and a choke assembly comprising an inletconnected to the outlet for the low density component; and theseparation system is operable at wellhead pressure to remove well debrisentrained in the fluids to produce a solids rich phase and a fluidphase, the outlet for the low density component used for removing thefluid phase.